This invention relates generally to a process for the preparation of alumina. More specifically, the present invention is concerned with a process for the preparation of alumina having desirable pore diameter and surface area suitable for use as a catalyst carrier.
Alumina is widely used as a catalyst carrier because of its higher mechanical strength and larger surface area as compared with other inorganic oxides, a large surface area being long considered to be advantageous since the reactivity of a catalyst depends on its surface area. In recent years, however, the pore diameter and pore distribution of a catalyst carrier have been recognized as being also of importance. In fact, the pore diameter and the pore volume of a catalyst have a greater influence on the catalytic reaction than the surface area when the molecular size of the reactant has an effect on the catalytic reaction. In addition, the mechanical strength of a catalyst generally depends upon its pore diameter and pore volume. Therefore, many attempts have been made to provide an effective method which can control the pore distribution of alumina and which can produce an alumina carrier having an improved mechanical strength.
In many catalytic reactions, the pore diameter of the catalyst has an important effect on the catalytic activity and selectivity. The smaller the pore diameter, the lower becomes the rate of diffusion of the reactant molecules into the catalyst pores, resulting in a decrease of the catalytic effectiveness factor and, thus, the catalytic activity. When the pore diameter is increased, the catalytic effectiveness factor is also increased, but the increase of the effectiveness factor stops after the pore diameter reaches a certain value. When the pore diameter is increased beyond the specific value, the apparent catalytic activity is decreased as a result of a decrease in the surface area. If the pore diameter is increased while maintaining the surface area within a certain level, then the pore volume becomes so large that the mechanical strength of the catalyst is considerably deteriorated. Therefore, in order to provide a catalyst exhibiting excellent catalytic activity, it is necessary to provide a catalyst carrier which has both an optimum pore diameter and large surface area while controlling the pore volume to a value so that the deterioration of the mechanical strength is prevented.
Among many types of alumina, .gamma.-alumina is known to have a high thermal stability and a high mechanical strength. It is also known that .gamma.-alumina can be produced by calcining boehmite gel and that .gamma.-alumina can be converted into alumina of other crystalline forms such as .alpha.-alumina. Boehmite gel is a hydrated gel of fibrous boehmite crystallites, generally called "pseudo-boehmite". The boehmite gel can be generally produced by aging non-crystalline aluminum hydroxide at a temperature of at least 50.degree. C. and a pH of 6-11. To produce alumina carriers (not only .gamma.-alumina but also other forms of alumina) having suitably controlled pore diameter distribution and pore volume, the crystal size of the pseudo-boehmite must be adjusted to a suitable size. When the pseudo-boehmite has an excessively large crystal size, the resultant alumina formed by calcination of the pseudo-boehmite will have a large pore diameter. On the other hand, when the pseudo-boehmite has an excessively small crystal size, the resulting alumina will have a small pore diameter and, further, the pore volume will be reduced as a result of the excessive sintering of the crystallites during calcination. Furthermore, if the pseudo-boehmite crystallites do not have uniform sizes, the resultant alumina obtained by calcination will have non-uniform crystal sizes so that the pore diameter will be also non-uniform and the pore volume will become very small. Additionally, when the boehmite gel contains a large amount of non-fibrous fine crystals or amorphous components, the gel will densely agglomerate and the alumina product obtained therefrom by calcination will have a small pore volume. Therefore, in order to obtain alumina carriers having desirably controlled pore diameter distribution and pore volume, it is necessary to prepare a boehmite hydrogel containing alumina crystallites of a uniform and suitable size.
Conventionally, the control of the pore diameter and pore volume of a catalyst carrier has resorted to a method in which the particle size of each of the primary particles constituting the carrier and the packing of the primary particles are controlled so as to control the size and the volume of the space defined between the primary particles. However, from the standpoint of mechanical strength, the manner of packing cannot be freely varied but is limited according to the particle size. In other words, the pore diameter cannot be varied independent of the pore volume. This also applies to alumina carriers. Thus, though the pore diameter can be increased by increasing the particle size of the primary particles, the pore volume cannot be increased and the specific surface area is reduced thereby.
Various methods have thus far been proposed for preparing alumina, especially .gamma.-alumina, having a large pore volume and a large pore diameter while maintaining the specific surface area at a high level. One such method includes controlling the shrinkage of the gel structure during drying and calcining of boehmite hydrogel. Since, according to this method, the specific surface area is maintained unchanged, the control of the pore volume can be made by the control of the pore diameter. An example of the above method is disclosed in Journal of Polymer Science, Vol. 34, p. 129, in which the drying speed of the boehmite hydrogel is controlled. This method, however, suffers from a drawback because the control of the pore volume must be limited to a very narrow range in order to maintain the mechanical strength of the alumina product in an appropriate range. Some methods are proposed which are capable of controlling the pore volume in a wide range, such as (1) a method in which a water-soluble polymeric material such as a polyethylene glycol is added to the boehmite hydrogel (Japanese Published Unexamined Patent Applications Nos. 52-104498 & 52-77891); and (2) a method in which an alcohol is substituted for a part of, or a greater part of, the water in the boehmite hydrogel (Japanese Published Unexamined Patent Application No. 52-123588). In both methods, the pore volume is controlled by use of an amount of the water-soluble polymeric material (in the former case) or the alcohol (in the latter case) which may inhibit the dense aggregation of the boehmite crystallites that would occur during the drying step as a result of the surface tension of the water contained in the gel. The alumina carrier obtained by these methods, however, fails to exhibit satisfactory mechanical strength and stability to water because the binding forces between boehmite crystallites are weak due to the deterioration of the surface tension of water.
Japanese Examined Patent Publication No. 49-37517 proposes a method in which a part of the boehmite gel is first changed to xerogel and the xerogel is then incorporated into a hydrogel of boehmite to increase the pore volume. The alumina thus obtained has a so-called "double-peak" pore distribution having small pores defined between boehmite fine crystallites and large pores defined between the xerogels. Therefore, this method cannot produce an alumina carrier having a large pore volume in pores having a desirable pore diameter and a large surface area.
In order to control the particle size of the primary particles forming an alumina carrier, it is necessary to control the particle size of the primary particles forming the boehmite hydrogel which is a precursor material for the carrier. As described previously, the conventional method of preparing a boehmite hydrogel includes aging seed aluminum hydroxide at a pH of 6-11 which range is suited for the formation of boehmite. However, in such a pH range, the rate of dissolution of fine crystallites is extremely low so that the so-called Ostwald's rate (rate at which crystals grow with accompanying dissolution of fine crystallites) becomes very low. Therefore, the conventional method requires a long period of time for the growth of boehmite particles.
U.S. Pat. No. 4,248,852 discloses a method for the preparation of an alumina carrier, especially .gamma.-alumina, having a large surface area and a controlled pore volume. This method includes alternately adding to a slurry containing aluminum hydroxide which serves as seed crystals, while maintaining the temperature of the slurry at 50.degree. C. or more, an aluminum compound and a neutralizing agent with stirring so as to form active aluminum hydroxide which is occluded into the seed aluminum hydroxide, thereby to accelerate the growth of the crystals. The thus grown boehmite particles combine with each other to form a sparse aggregate. By controlling the state of the aggregate, the shrinkage of boehmite gel during drying can be prevented and an alumina carrier having a controlled pore characteristics and a large surface area can be obtained. This method, however, has a problem in practice because the operation of the process is complicated.